<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Document</title>
  </head>
  <body>
    <script>
      class Father {
        constructor(x, y) {
          this.x = x;
          this.y = y;
        }
        sum() {
          console.log(this.x + this.y);
        }
      }
      class Son extends Father {
        constructor(x, y) {
          super(x, y);		// 调用父类的构造方法
        }
      }
      var son = new Son(1, 2);
      son.sum();			    // 输出结果：3
    </script>
  </body>
</html>